1. Field for the Invention
The present invention relates to an arrangement of compensating for temperature dependent characteristics of a surface acoustic wave (SAW) filter. An SAW filter, with which the present invention may be employed as concerned, is an intermediate frequency (IF) band-pass filter provided in a first IF stage of a double-superheterodyne receiver.
2. Description of the Prior Art
As is well known, a superheterodyne receiver is a receiver in which all incoming modulated radio-frequency carrier signals are converted, using a heterodyne action, to a common IF carrier value for additional amplification and selectivity prior to demodulation.
On the other hand, a double-superheterodyne receiver is a type which utilizes two frequency converters before final detection.
A double-superheterodyne receiver, as commonly used in a mobile telecommunications systems or the like, comprises a first IF stage in which a first mixer generates a relatively high IF (by way of example, 90 MHz). In order to deal with such a high intermediate frequency, a crystal filter has been found to be suitable in that a highly selective filtering can be expected.
However, such a crystal filter is expensive and requires cumbersome adjustments when both manufactured and installed. As a consequence, it is a current practical tendency to use an SAW band-pass filter in place of a crystal filter in the first IF stage of a double superheterodyne receiver. That is to say, a SAW filter is inexpensive and easy to install. A SAW filter consists of a piezoelectric bar with a polished surface along which surface acoustic waves can propagate.
As mentioned above, an SAW band-pass filter is found advantageous in connection with cost and performance and easy installation into a receiver, but a difficulty has been encountered in that an SAW filter's performance characteristics tend to be overly susceptible to ambient temperature changes.
Before describing the present invention, a known double superheterodyne reception arrangement using a SAW intermediate filter will be discussed with reference to FIG. 1.
It should be noted that, throughout the instant specification, each signal and corresponding frequency thereof will be denoted by corresponding reference characters for the sake of convenience.
As shown in FIG. 1, mixer 10 in a first IF stage is supplied with a radio frequency (RF) modulated carrier signal Fa through an input terminal 12. The input signal Fa (by way of example, 900 MHz) is frequency converted at the first mixer 10 by multiplication with a locally generated signal (by way of example, 990 MHz) outputted from a first local oscillator 14. An output Fc of the first mixer 10 is then filtered by an SAW type band-pass filter 16 whose center frequency is denoted by Fd.
The first local oscillator 14 takes the form of frequency synthesizer and includes a reference oscillator 18 which generates a signal which exhibits suitable frequency accuracy and stability to satisfy system requirements. An output of the reference oscillator 18 is applied to a phase detector 20 which also receives a signal from a variable frequency divider (viz., programmable counter) 22. The phase detector 20 compares the phases of the two signals applied and generates an error signal which is proportional to the phase difference between said two signals. The error signal is filtered by a loop filter (low-pass filter (LPF)) 24 which smoothes and shapes it into a voltage suitable for controlling a voltage controlled oscillator (VCO) 26. The output of the VCO 26, denoted by Fb, is applied to the first mixer 10 and also split and fed back to the programmable counter 22 by way of a prescaler 28. When the output of the low-pass filter 24 is applied to the VCO 26, the output frequency of the VCO 26 is induced to change in a direction to establish a constant phase difference (typically "zero") between the two signals applied to the phase detector 20. The arrangement of the first local oscillator 14 is well known in the art.
The output of the first IF band-pass filter (SAW band-pass filter) 16, denoted by Fe, is subject to second frequency conversion at a second mixer 30 via multiplication with a local frequency Ff (by way of example, 89.545 MHz) applied from a second local oscillator 32. The output of the second mixer 30, denoted by Fg (by way of example, 545 KHz), is applied to external circuitry through a second band-pass filter 34, an amplifier 36 and an output terminal 38. The local oscillator 32 is a high precision oscillator such as a crystal controlled oscillator.
As mentioned previously, the SAW band-pass filter 16 inherently has performance characteristics which undesirably vary with ambient temperature changes. FIG. 2 is a graph which shows (in ppm) the manner in which the center frequency Fd of the SAW filter 16 deviates with respect to ambient temperature. As shown by curve A, the degree by which the center frequency Fd deviates, increases as an ambient temperature varies above and below about room temperature (for example).
FIG. 3 compares curve A, which is the same as that shown in FIG. 2, with curve B, which is a plot of frequency deviations (in ppm) exhibited by the first intermediate frequency Fc, with respect to ambient temperature changes. It should be noted, however, that the first intermediate frequency Fc is shifted or deviated very little, if at all, with respect to ambient temperature changes as shown in FIG. 3.
As will be appreciated the frequency difference between the two curves A and B increases as the temperature increases and decreases from a predetermined temperature range.
It is assumed that the maximum frequency difference between the two curves A and B corresponds to a bandwidth .delta.W1 (FIG. 3). As a consequence, the pass-band of the SAW band-pass filter 16 must be augmented in accordance with the bandwidth .delta.W1 as shown in FIG. 4. In this figure (a) "Fd'" denotes a center frequency of the SAW band-pass filter 16 (viz., a center frequency of a given channel) and (b) "Fx" denotes a center frequency of an adjacent channel.
More specifically, in the event that an originally planned bandwidth Wor of a given channel is not expanded, the desired incoming channel will be contaminated by the adjacent channel Fx. In other words, according to the known arrangement shown in FIG. 1, due to the temperature response characteristics of the SAW filter, the channel spacing between each two adjacent channels must be undesirably increased in order to obviate interchannel interference.